1. Field of the Invention
The present invention is related generally to a method and an apparatus for correcting a mounting error when mounting a workpiece to a machine tool by use of a fixture and more particularly to a method of and an apparatus for correcting a mounting error, which are suited to a simultaneous quinaxial controlled machine tool for working a configuration of the workpiece at a high degree of freedom, especially, a machine tool structured to have rotational control axes on the side of a workpiece table.
2. Description of Related Background Art
A fixture (jig) for fixing the workpiece is employed for facilitating machining by a machine tool. In this case, a mounting error (offset) of the workpiece is produced in the machine tool due to the use of the fixture, and it is therefore required that such a mounting error be corrected. This correction is known as an offset correction.
According to a conventionally employed fixture offset correcting method, a set offset quantity is corrected with respect to positions in three axial directions x, y, z. This offset correcting method is referred to as a simple fixture offset correcting method.
The offset quantity based on this method is given in the form of triaxial (X, Y, Z) vector quantities (.DELTA.X, .DELTA.Y, .DELTA.Z). A coordinate system of the offset vectors is based on directions on a mechanical coordinate system. For this reason, a correcting calculation is quite simple.
There arise, however, a variety of the following problems inherent in the simple fixture offset correcting method.
First, the mounting errors of the workpiece with respect to the fixture may be caused not only in the three directions X, Y, Z but also in three rotational degree-of-freedom directions, or yaw, roll and pitch. There exists, however, no correcting method coping therewith.
Second, when rotating the workpiece, x-, y- and z-axis directional positions shift, with the result that the direction of error vectors also change. Hence, the error can not be properly corrected with the same set value. The proper correction entails setting an offset numerical value at each angle by giving an offset vector corresponding to each angle every time rotary axes A, B are commanded. In this case, even a quinaxial machine tool, if cutting within that angle, performs correction by effecting the dividing, usable by previously calculating and setting offset vectors corresponding to all the dividing angles.
Third, the simple fixture offset correcting method is not usable under simultaneous quinaxial control wherein a rotating angle of the workpiece is changed during machining.
Consequently, a complete fixture offset correcting method is required which means that all six degree-of-freedom offsets be properly corrected. This method, however, presents problems, such as numerous variables making the arithmetic operation complicated and quick control impossible, and also requiring a part program to be rewritten.